$-10de - 3e + f - 1 = -7e + 2f - 7$ Solve for $d$.
Combine constant terms on the right. $-10de - 3e + f - {1} = -7e + 2f - {7}$ $-10de - 3e + f = -7e + 2f - {6}$ Combine $f$ terms on the right. $-10de - 3e + {f} = -7e + {2f} - 6$ $-10de - 3e = -7e + {f} - 6$ Combine $e$ terms on the right. $-10de - {3e} = -{7e} + f - 6$ $-10de = -{4e} + f - 6$ Isolate $d$ $-{10}d{e} = -4e + f - 6$ $d = \dfrac{ -4e + f - 6 }{ -{10e} }$ Swap the signs so the denominator isn't negative. $d = \dfrac{ {4}e - {1}f + {6} }{ {10e} }$